This application relates to turbine combustion, and in particular relates to a rich-quench-lean turbine combustor with low NOx and CO emissions.
Over the past ten years there has been a dramatic increase in the regulatory requirements for low emissions from turbine power plants. Environmental agencies throughout the world are now requiring low rates of emissions of NOx, CO and other pollutants from both new and existing turbines.
Traditional turbine combustors use non-premixed diffusion flames where fuel and air freely enter the combustion chamber separately. Typical diffusion flames are dominated by regions that burn at or near stoichiometric conditions. The resulting flame temperatures can exceed 3000.degree. F. (1650.degree. C.). Because diatomic nitrogen reacts rapidly with oxygen at temperatures exceeding about 2850.degree. F. (1565.degree. C.), diffusion flames typically produce relatively high levels of NOx emissions.
One method commonly used to reduce peak temperatures, and thereby reduce NOx emissions, is to inject water or steam into the combustor. Water or steam injection, however, is a relatively expensive technique and can cause the undesirable side effect of quenching carbon monoxide (CO) burnout reactions. Additionally, water or steam injection methods are limited in their ability to reach the extremely low levels of pollutants now required in many localities.
Another method to reduce NOx emissions is by utilizing a rich-quench-lean (ROL) gas turbine combustor. In a rich-quench-lean combustor, a combustor is divided into a fuel rich stage, a quench stage and a fuel lean stage. In the fuel rich stage, (rich meaning an equivalence ratio .O slashed.&gt;1), a fuel-air mixture is partially burned because the fuel-air mixture is introduced with an insufficient amount of air to complete combustion. [Note that equivalence ratio is fuel/air ratio normalized by the stoichiometric fuel/air ratio, .O slashed.=1 for stoichiometric conditions, .O slashed.&gt;1 for fuel rich conditions, and .O slashed.&lt;1 for fuel lean conditions.] Fuel rich combustion is desirable because a large portion of any bound nitrogen species (for example, NH.sub.3) in the fuel will be converted into N.sub.2 during combustion within the rich stage. By converting the reactive bound nitrogen species to relatively non-reactive N.sub.2, emissions of NOx are reduced.
Next, additional air, termed in the art to be "quench air", is added downstream from the rich stage to complete combustion within a lean stage. If the quench air is not uniformly and rapidly introduced, however, high NOx levels will be produced in local regions of the combustor due to high temperatures. Although rapid mixing can be achieved with a high pressure drop, this reduces the overall efficiency of the turbine.
Therefore, it is apparent from the above that there exists a need in the art for improvements in rich-quench-lean combustor design to achieve rapid mixing of quench air and rich stage burned gas while maintaining low emission levels and low pressure drop across the quench stage.